Strongly acidic cation exchangers can be obtained by functionalizing crosslinked styrene bead polymers. The functionalization generates covalently bonded sulfonic acid groups through reaction of aromatic units of the polymer skeleton with a sulfonating agent, for example sulfuric acid.
One problem with the known strongly acidic cation exchangers is that of their stability under stress, which is not always sufficient. For instance, cation exchanger beads can break up as a result of mechanical or osmotic forces. For all applications of cation exchangers, the exchangers present in bead form must maintain their habit and must not be degraded partly or even entirely during the application or break up into fragments. Fragments and bead polymer splinters can get into the solutions to be purified during the purification and contaminate them themselves. Moreover, the presence of damaged bead polymers is unfavorable for the functioning of the cation exchangers themselves which are used in column processes. Splinters lead to an elevated pressure drop of the column system and hence reduce the throughput of the liquid to be purified through the column.
A further problem of the known strongly acidic cation exchangers is their tendency to release sulfonated, water-soluble fragments from water in use as a result of the action of a wide variety of different oxidizing agents dissolved in water (atmospheric oxygen, hydrogen peroxide, vanadyl salts, chromates). This phenomenon, known in general to those skilled in the art by the term “leaching” leads to the enrichment of sulfonated organic constituents in the water to be treated, which can lead to various problems in the downstream systems which are reliant on the supply of fully demineralized water. For example, the water-soluble fragments lead, for example, to corrosion problems in the cooling circuit of power plants, to defects in the microchips produced in the electronics industry, to the failure of the system owing to excessively high electrical conductivity of the water in eroding machines.
There has been no lack of attempts to provide strongly acidic cation exchangers which have an improved mechanical stability, osmotic stability and/or an improved oxidation stability.
For instance, the mechanical and osmotic stability of strongly acidic cation exchangers can be increased via two-stage structure of the bead polymers, in a so-called seed-feed process. Such processes are described, for example, in EP 0 101 943 A2, EP-A 1 000 659 and DE-A 10 122 896. The cation exchangers produced in two stages exhibit an outstanding mechanical stability, but release significantly more sulfonated degradation products to the treated water than the resins produced in one stage.
The incorporation of small amounts of acrylate monomers into the styrene-divinylbenzene copolymer also leads to an increased mechanical stability, as described in U.S. Pat. No. 4,500,652 and DE-A 3 230 559. However, the presence of acrylic acid units in the polymer structure leads to a higher oxidation susceptibility of the resins.
Bachmann et al. describe in EP-A 868 444, a process for producing mechanically and osmotically stable cation exchangers by sulfonating without addition of chlorinated swelling agents at temperatures between 125 and 180° C. However, dispensing with the chlorinated swelling agent in the sulfonation does not improve the oxidation stability of the resins.
The oxidation stability of the strongly acidic cation exchangers can be increased by adding antioxidants, as described, for example, in EP-A 0 366 258. These antioxidants are washed slowly out of the resin, which leads to the release of organic constituents to the water treated. Furthermore, they are spent after a relatively short time in use. Thereafter, the resin treated with antioxidants exhibits the same oxidation sensitivity as a conventional strongly acidic cation exchanger.
The oxidation stability of the strongly acidic cation exchangers can also be improved by increasing the crosslinking density of the bead polymer. However, the incorporation of major amounts of crosslinker makes the polymers more brittle, which leads to a significant reduction in the mechanical stability of the beads. Moreover, the kinetics of the cation exchange decrease significantly with increasing crosslinking density, which leads to insufficient absorption capacities in many applications.
There is thus still a need for cation exchangers with rapid exchange kinetics, high mechanical and osmotic stability, and simultaneously high oxidation stability.
The problem addressed by the present invention is therefore that of providing a simple, robust and economically viable process for producing cation exchangers with rapid exchange kinetics, high mechanical and osmotic stability, and high oxidation stability.